The aim of the study was to determine the radiosensitivity and the growth kinetics of pulmonary micrometastases of the R1H tumour of the rat. Lung metastases were induced by intravenous injection of viable tumour cells. At different time intervals (3-32 days) after injection, lungs were locally irradiated with 200 kVp X-rays, using 1.5 Gy/fraction. Total doses of 6-33 Gy were administered within 11 days. Endpoints used were survival time, local control rate, and number of metastases in the lungs at autopsy. The data were evaluated using the multi-target model. Beginning in the fifth week after tumour cell inoculation the animals started to exhibit a pronounced dyspnoea and were sacrificed. Sections revealed an extensive metastatic infiltration of the lungs. With increasing total dose a prolongation of survival time as well as an increase in cure rate was observed. The number of metastases found in the lungs decreased with increasing total dose. It is concluded that metastatic growth does not start earlier than 3 days after tumour cell inoculation and accelerates continuously. The doubling time of the tumour cells in the micrometastases decreases continuously and from 5.2 to 1.2 days between day 3 and 40, whereas larger metastases containing more than 10(6) cells show gompertzian growth kinetics. The cell doubling time in this stage of metastatic growth is longer than 5 days. During the first 4 weeks of metastatic growth the radiosensitivity of metastatic R1H cells in the lungs is the same as in vitro.
